Mineral-coated paper products and methods for making them



United States Patent 3,081,198 MINERAL-COATED PAPER PRODUCTS AND METHODSFOR MAKING THEM Edgar W. Miller, Levittown, Pa., assignor to Rohm & HaasCompany, Philadelphia, Pa., a corporation of Delaware No Drawing. FiledApr. 13, 1960, Ser. No. 21,889 13 Claims. (Cl. 117-155) This inventionrelates to mineral-coating compositions and to paper coated therewith.Mineral-coating compositions, such as are commonly applied to paper bodystock in the manufacture of mineral-coated paper and the like, compriseaqueous suspensions of finely divided mineral matter, referred to hereinas pigment, such as clay, calcium carbonate, blanc fixe, finely dividedmetals such as aluminum, color lakes, tinctorial oxides, or the like andan aqueous dispersion or solution of an adhesive such as casein, glue,starch, or the like.

In general, mineral coatings are applied to paper to improve theappearance, the printing qualities, or other properties of the paper.The mineral-coating covers the individual fibers of the paper surfaceand fills interstices between fibers, thus rendering the surface of thepaper more level and more uniform in texture. It is primarily thepigment content of the coating composition which provides the desirablequalities of the coating, whereas the adhesive provides chiefly thefunction of suitably binding the mineral matter to the paper; e.g. sothat it will not be removed by the pull of printing ink during theprinting operation. Nevertheless, the particular adhesive used does haveconsiderable influence upon the working qualities of the coatingcomposition; e.g., viscosity, flow, spreadability, etc. Likewise, theadhesive used in the coating composition has a definite effect upon thequality and appearance of the finished coated paper made therewith.

For example, the plasticity of the adhesive has a pronounced effect onthe ability of the supercalender to produce a level and good printingsurface.

It has heretofore been suggested to employ various pigmented aqueousdispersions of such waterinsoluble synthetic resinous polymers aspolymerized methyl acrylate, ethyl acrylate, or butyl acrylate, ormethyl or butyl acrylate copolymerized with acrylonitrile or ethyl,methyl or butyl methacrylate. found to be insufliciently adherent to thepaper to reliably resist being pulled up by the ink during printingoperations. United States Patent 2,790,735 discloses and claims coatedpapers obtained by the application of pigmented aqueous dispersionscontaining 8 to 25%, on the weight of the pigment of water-insolublecopolymers of "a lower alkyl acrylate with 4 to 7% of an acid, or saltthereof, selected from methacrylic and acrylic acids. United StatesPatent 2,790,736 discloses and claims coated papers obtained by theapplication of pigmented aqueous dispersions containing 8 to 25%, on theweight of the pigment, of water-insoluble copolymers of a lower alkylacrylate with 2.5% to 7% of an acid, or a salt thereof, selected fromcertain monoethylenically unsaturated polybasic acids, such as itaconic,aconitic, the dimer of methacrylic acid, the trimer of methacrylic acid,maleic acid, and fumaric acid. A copending United States patentapplication Serial No. 541,072, filed October 17, 1955, now

Patent No. 2,874,066, discloses and claims coated papers obtained by theapplication of pigmented aqueousdispersions con-tammg 8 to 25 on theweight of the pigment,

' of water-insoluble copolymers of 2.5 to 7% by weight of an unsaturatedamide, such as acrylamide or methacrylamide. Aqueous dispersions of thevarious water-insoluble copolymers can be satisfactorily applied inlarge excess by an applicator roll rotating in such a direction However,such polymers have been.

"ice

that the surface travels opposite to the direction of travel of thesheet followed by a slowly rotating metering rod to limit the thicknessof the coating. Another system involves applying excess and blowing ofithe excess by what is called an air-knife. However, thesewater-insoluble polymer dispersions cannot be satisfactorily applied bycertain conventional coaters in which the pigmented coating is appliedas a high-solids, high-viscosity system to the first of a train oftransfer rolls on which the composition is worked as it passes from onenip to the next until it is finally brought into contact with the papersheet by contact therewith of the last roll of the train. This lattertype of coating equipment has the advantage of high speed, especiallywith respect to drying since the highsolids system contains less solventor vehicle to be removed during drying. However, the water-insolublepolymer dispersions produce an irregular pattern which is commonlyreferred to by the term turkey-tracks.

In accordance with the present invention, it has beencoon r in which nis an integer having a value of 1 to 2, with 60 to of at least one esterof an acid of the formula with an alcohol having from 1 to 4 carbonatoms.

The preferred copolymers of the present invention are those havingcomparatively low molecular weights from about 50,000 up to about300,000; but copolymers having higher molecular Weights even up to aboutone million can be used but require special care because of their highviscosity after neutralization.

The copolymers may be made by various procedures. However, in general,an emulsion process, employing an appropriate amount of a chainregulator to produce the preferred low molecular weight copolymers, ismost suitable. In general, the polymerization may be efiected byintroducing a mixture of the several monomers in appropriateproportions, a free-radical polymerization initiator, and, for makingthe preferred copolymers, a chain regulator into an aqueous solution ofan emulsifying and/ or dispersing agent.

The initiator may be an organic peroxide such as benzoyl peroxide,acetyl peroxide, lauroyl peroxide, or it may be an azo compound such asdiisoazobutyronitrile.

The amount of the initiator may vary from about 0.5% to 6% by weight ofthe total weight of the monomers. The amount of the chain regulator mayvary from 0.1% to 1.5% more or less depending upon the particular chainregulator. Examples of chain regulators include long-chain alkylmercaptans, e.g. t-dodecyl mercaptan, octyl mercaptan, octadecylmercaptan, alcohols,

such as isopropanol, isobutanol, or higher alcohols, such as t-octylalcohol and lauryl alcohol, carbon tetrachloride,

ethylene tetrachloride C Cl bromotrichloromethane, and

substituted ally-l chloride, such as methallyl chloride.

The following example illustrates the preparation of the copolymersalts. The parts and percentages are by weight unless otherwise noted.

EXAMPLE A An aqueous emulsion is prepared by agitating together in aseparate vessel the following:

To the reaction vessel, there are then added 450 grams of the monomeremulsion, the temperature of the contents being then adjusted to 20 C.,and 0.6 gram of sodium metabisulfite is then added. The temperaturerises and levels off at about 50 C. Then additional monomer emulsion isadded at a rate of 222 grams every ten minutes and concurrently asolution of 3.75 grams of sodium metabisulfite in 150 cc. of water isadded at a rate of 15 cc. every ten minutes until all of the emulsionand metabisulfite are added. The total time of polymerization (duringwhich the temperature is maintained at 50 to- 56 C. by cooling) amountsto 110 minutes at which time about ten drops of t-butyl hydroperoxideare added and the reaction mixture is stirred another half hour. Aftercooling to 30 C., the dispersion is filtered. The filtrate provides anapproximately 40% solids emulsion copolymer dispersion (substantially100% yield) in which the copolymer composition is approximately 25%methacrylic acid, 50% methyl methacrylate, and 25% ethyl acrylate.

The polymer of the present invention is best incorporated in the coatingcomposition by first adding all other binders which are to be used tothe Water of dilution. Then, the aqueous dispersion of thewater-insoluble acid polymer (prepared as in Example A) is added andfollowed by the addition of an aqueous dispersion of the variouspigments. The acid polymer is then solubilized in situ by adding about 1to 2 equivalents of ammonium hydroxide or of a volatile amine, based onthe acid content of the polymer. If casein, soya-p-rotein, or some otheralkali-dispersed binder is used, it is especially desirable to add theacid-containing emulsion polymer last. Neutralization of the added acidpolymer occurs rapidly on addition to the alkaline binder dispersion.

Instead of ammonia, aqueous solutions of volatile, water-soluble aminesmay be used in forming the binder dispersion or solution. Such aminesinclude diethylamine, triethylamine, ethanolamine, diethanolamine,.triethanolamine, and morpholine.

The amount of the copolymer salts of low molecular weight included inthe pigmented mineral coating compositions may vary widely from as lowas about 5% by weight of the total binder content therein to as high as65% by weight thereof. The amount of the binder in such coatings is from8 to 25 by weight of the pigment, and is preferably 12 to 20% by weightthereof.

The other component or components of the binder of the mineral coatingcomposition may be a protein soluble in the alkaline dispersion or itmay be a synthetic linear addition polymer which is insoluble in theaqueous alkaline system. Examples of the proteins that may be usedinclude casein, soya-protein, soybean protein, or the like. The polymersmay be any natural, artificial, or synthetic rubber latex or an aqueousemulsion polymer of isobutylene, butadiene, ethylene, vinyl acetate,vinyl chloride, vinylidene chloride, acrylonitrile, meth-acrylonitrile,esters of acrylic acid or methacrylic acid such as those of methylacrylate, ethyl acrylate, butyl acrylate, and copolymers thereof, withmethyl, ethyl, or butyl methacrylate. A desirable characteristic ofthese synthetic polymers is the apparent second order transitiontemperature value thereof which is preferably from 45 C. to +20 C.except in the case of copolymers formed largely of vinyl acetate, suchas copolymers of 70 to 93% vinyl acetate, 5 to 25% ethyl or methylacrylate, and 2 to 8% of acrylic, methacrylic, or itaconic acid.

This second order transition temperature is herein referred to as the Tvalue and is defined as the transition temperature or inflectiontemperature which is found by plotting the modulus of rigidity againsttemperature. A convenient method for determining modulus of rigidity andtransition temperature is described by I. Williamson, British Plastics23, 87-90, 102 (September 1950). The T value here used is thatdetermined at 300 kg./cm.

The other component or components of the binder may also be any of thewater-insoluble and alkali-insoluble copolymers having an apparentsecond order transition temperature (T value) of 45 C. to +20 C. anddescribed in the patents or applications mentioned hereinabove, namelyUnited States Patents 2,790,735, 2,790,736, and 2,874,066.

When used with the proteins, it has been found that the copolymer saltsof the present invention surprisingly increase the wet-rub resistance ofthe coatings obtained. Many of the copolymer salts which contain a largeproportion of an acrylic acid ester, such as ethyl acrylate, butylacrylate, and so on, plasticize the protein. When the copolymer salts ofthe present invention are dissolved in the aqueous phase of the aqueousdispersions of the polymers mentioned hereinabove, whether the aqueousdispersions are natural, as in natural rubber latex, or of syntheticorigin, it has been found that the compositions obtained can be appliedto the paper surfaces as a high solids, high viscosity system by meansof the conventional coaters comprising a train of transfer rolls Withoutgiving rise to the turkey-tracks commonly resulting from the applicationof aqueous dispersions of water-insoluble polymers. In addition, thepresence of the copolymer salt of the present invention imparts theimproved resistance to picking especially when the other component ofthe binder is a Water-insoluble polymer which contains neithercarboxylate salt groups nor amide groups.

In general, the compositions may be prepared by mix- I ing the copolymersalt of the present invention, preferably in the form of an aqueoussolution thereof, with the other binder, preferably in the form of anaqueous solution as in the case of the protein or as an aqueousdispersion in the case of the insoluble polymer.

The pigment or pigments are preferably mixed and dispersed in a smallamount of water before mixing with the copolymer dispersion. When clayis used as a part of the pigment, and in preferred embodiments it formsa predominant proportion of the pigment, the dispersion is preferablyadjusted to a pH of 8.5 to 9.5 to obtain the optimum dispersion of theclay.

After mixing the pigment and the copolymer dispersions, the resultingcoating composition is applied to the paper or paperboard at a totalsolids concentration of at least 40% and preferably 50 to 60% by anysuitable equipment such as immersion roll and doctor system, gravureroller system, brush coater, spray coater, or conventional transfer rollcoater as described hereinabove. It may be applied to the paper afterdrying and/or conditioning. Alternatively, it may be applied during thefirst drying operation on the paper where it has undergone only partialdrying. For example, the coating system may be mounted at anintermediate point in the drier on the papermaking machine, such as at apoint where the paper has been reduced to approximately 50% moisturecontent.

After the coating operation, the coated sheet is dried and may then becalendered, and subsequently printed. The drying may be the usual typeprovided in which air at about 230 to 260 F. to C.) is directed againstthe paper for 30 to 45 seconds. The paper and coating may reach atemperature of about F. (ca. 85 C.) during the drying operation.Printing may be effected by the conventional inks of precipitation typeor heat-setting type including those based on drying oils. The coatedproducts of the present invention are receptive to single color inks andmulti-color inks of graded vis cosity and are able to withstand the pullof such inks. It may be overcoated, after printing, with wax, lacquer,or other compositions.

The pigments that may be employed include clays, especially of thekaolin type, calcium carbonate, blanc fixe, talc, titanium dioxide,colored lakes and toners, ochre, carbon, black, graphite, aluminumpowder or flake, chrome yellow, molybdate orange, toluidine red, copperphthalocyanines, such as the Monastral blue and green lakes. The termminera in the claims is intended to cover all such types of pigmentarymatter whether of strict mineral character or partly of organicmaterial.

The compositions are adapted to be satisfactorily applied withoutirregular pattern-development by a wide variety of equipment includingthose involving a train of transfer rolls for working the composition asit is fed from nip to nip to the point of contact with the paper. Thecoated papers are highly resistant to pick and, surprisingly, towet-rubbing.

In the following examples, which are illustrative of the presentinvention, the parts and percentages are by weight unless otherwisenoted. The ink numbers referred to designate inks of the Institute ofPrinting having graded tackiness increasing from No. 1 to No. 6. Thenumerical values given under Pick Resistance refer to the linear speed(in feet per minute of the sector in an I.G.T. Pick Resistance Tester)at which coating failure occurs when using the ink referred to. Thehigher the value, the greater the resistance. In the wet-rub test usedin the examples, the coated paper to be tested is lapped over a piece ofglossy black paper so that a substantial area of the black paper isexposed and extends beyond the edge of the coated paper to be tested anda drop of water is applied to the coating to be tested. The index fingerof the operator is drawn from the wet spot on the coating and onto theblack paper with light pressure, and the procedure is repeated tentimes. The water on the black paper is allowed to evaporate, and thereflection of light from the spot thereof to which any pigment istransferred from the coated sheet by the finger is measured. The higherthe reflection of light, the poorer is the wet-rub resistance. Thevalues given under Web-Rub Resistance refer to the brightness(reflectance) of the spot of coating rubbed off onto the black paper.Smaller values indicate better wet-rub resistance.

Example 1 (a) A series of coating compositions were made containingmixtures in various proportions of the emulsion polymer of Example Awith an emulsion copolymer (B) .of about 80% of ethyl acrylate, 3% ofitaconic acid, and 17% of methyl methacrylate, having a T value of about2 to 5 C. Table I following indicates the proportions of the copolymersin the respective mixtures. After mixing the two emulsion copolymers,water was added to reduce the solids concentration to 55%. A claydispersion having a pH of 9 and consisting of 100 parts of clay, 44parts of water, and 0.2% (on clay) of sodium hexametaphosphate was mixedwith the polymer dispersion in an amount to provide 10% of total polymersolids on clay. Then, 1.5 equivalents (based on acid content of thepolymers) of ammonium hydroxide was added.

The various coating compositions thus obtained were applied topaperboard by means of a train of transfer rolls, the last of whichcontacted thetravelling sheet of paper, no wiper, spreader, or excessremoving device being' applied after the transfer roll contactedthepaper. About 2 to 3 pounds of coating composition (dry weight) per1,000 sq. ft. of the board was then applied to one surface. The coatedpaper was dried in an oven by air heated at 185 F. for a period of 45 to60 seconds. It was then calcndered at room temperature. The coated boardshowed no evidence of irregular pigmentation (turkeytracking) and had agood, smooth surface highly receptive to ink and resistant to pick. Italso showed good resistance to wet rubbing as is shown in the appendedtable.

TABLE I Binder Composition (percent) Pick Coating No. Resistance Wet-RubNo. 6 Ink Resistance Polymer Polymer of (B) Example A (b) Similarcoating compositions were presented in which the emulsion copolymer (B)was replaced with a copolymer of 95% of ethyl acrylate and 5% ofmethacrylamide and the copolymer of Example A was replaced with acopolymer prepared in the same fashion except that the monomercomponents consisted of 38% by weight of acrylic acid, 20% by weight ofethyl acrylate, and 42% by weight of 2-ethylhexyl acrylate. The coatingsshowed good pick resistance against a No. 4 ink and good wet-rubresistance.

Example 2 (a) The polymer of Example A was used in conjunction withsoya-protein as a binder. Coatings with various ratios of the twobinders were made up, using 16% of total binder based on pigment. Thecoatings were ap plied to paper as in Example 1; the test results arereproduced in Table II.

TABLE II Binder Composition (percent) Coating No. Wet-Rub ResistanceSoya- Polymer of Protein Example A (b) Coatings having comparablewet-rub resistance were obtained when the procedure of part (a) wasrepeated except that the soya-protein was replaced with casein.

(c) The procedure of Example 1 (a) is repeated substituting a copolymerof by weight of vinyl acetate, 5% by weight of ethyl acrylate, and 5% byweight of acrylic acid for the copolymer (B) of that example. Thecoatings exhibited good Wet-rub resistance and resistance to pick by aNo. 4'ink.

(d) The procedure of part (a) hereof is repeated substituting for thepolymer of Example A a copolymer prepared in the same fashion as ExampleA but from a mixture of monomers consisting'of 30% by weight ofmethacrylic acid, 25% by weight of methyl methacrylate, and 45% byweight of methyl acrylate. golgd wet-rub resistance and resistance topick by a No. 4 1n Example 3 binder weight on pigment. The results inTable III were obtained. I

The coatings showed.

(b) The procedure of part (a) hereof is repeated replacing the latex ofpolymer (C) with a natural rubber latex. The resulting product showedgood wet-rub resistance.

Example 4 The procedure of Example 1(a) is repeated with the samecopolymer (B) in conjunction with a copolymer of 15% of methacrylicacid, 45% methyl methacrylate, and 40% of ethyl acrylate, having aviscosity of 1.6 poises, at a concentration, used in place of thecopolymer of Example A. The proportions of copolymer (B) relative to themethacrylic acid copolymer were 40:60 parts by weight. Coatings obtainedfrom this compositlon showed good pick resistance since the failure inthe coating did not occur until a speed of 150 ft. per minute wasreached with a N0. 4 ink.

I claim:

1. A mineral-coated paper product comprising a paper sheet carrying on asurface thereof a dried coating of a composition comprising (1) afinely-divided pigmcntary material having a predominant proportion ofclay and (2) a binder comprising, for each 100 parts by weight of thepigment, from 8 to 25 parts by Weight of a mixture comprising (a) 5 to65% by weight of a water-soluble ammonium salt of a copolymer, having amolecular weight of at least about 50,000, of about to 40% by weight ofan acid of the formula in which n is an integer having a value of 1 to2, and 60 to 85% of at least one ester of an acid of the aforesaidformula with an alcohol having 1 to 2 carbon atoms and (b) 35 to 95% byweight of an alkali-soluble protein;

2. A product as defined in claim 1 in which the protein is casein.

3. A product as defined in claim 1 in which the protein is soya-protein.

4. A mineral-coated paper product comprising a paper sheet carrying on asurface thereof a dried coating of a composition comprising (1) afinely-divided pigmentary material having a predominant proportion ofclay and (2) a binder comprising, for each 100 parts by weight of thepigment, from 8 to 25 parts by weight of a mixture comprising (a) 5 to65% by weight of a water-soluble ammonium salt of a copolymer, having amolecular weight of at least about 50,000 of about 15 to 40% by weightof an acid of the formula in which n is an integer having a value of lto 2, and -60 to 85% of at least one ester of an acid of the aforesaidformula with an alcohol having 1 to 2 carbon atoms and (b) 35 to 95 byweight of a water-insoluble, alkaliinsoluble linear addition polymerselected from the group consisting of polymers having an apparent secondorder transition temperature from -45 C. to C. and polymers formedlargely of vinyl acetate.

5. A product as defined in claim 4 in which said linear addition polymeris natural rubber.

6. A product as defined in claim 4 in which said linear addition polymeris a synthetic rubber.

7. A product as defined in claim 4 in which said linear addition polymeris a polymer of at least one member selected from the group consistingof butadiene, styrene, isobutylene, ethylene, vinyl acetate, vinylchloride, vinylidene chloride, acrylonitrile, methacrylonitrile, andesters of an acid of the aforesaid formula with an alkanol having from 1to 8 carbon atoms.

8. A product as defined in claim 4 in which said linear addition polymeris a copolymer of at least alkyl acrylate in which the alkyl group has 1to 4 carbon atoms with up to 7% of a copolymerizable monoethylenicallyunsaturated acid.

9. A product as defined in claim 4 in which said linear addition polymeris a copolymer of at least one alkyl acrylate in which the alkyl grouphas 1 to 4 carbon atoms with up to 7% of a copolymerizablemonoethylenically unsaturated amide of an acid of the aforesaid formula.

10. A product as defined in claim 4 in which said linear additionpolymer is a copolymer of 70 to 93% by weight of vinyl acetate, 5 to 25%by weight of an alkyl acrylate in which the alkyl group has 1 to 2carbon atoms, and 2 to 8% by weight of an acid selected from the groupconsisting of acrylic acid, methacrylic acid, and itaconic acid.

11. A method of producing mineral-coated paper which comprises coatingat least one side of a paper sheet with an aqueous dispersion of 40 to70% total solids concentration, having a pH of 8.5 to 9.5, andcontaining a pig mcnt having a predominant proportion of clay and 8 to25%, on the total weight of pigment, of a binder comprising, for each100 parts by weight of the pigment, from 8 to 25 parts by weight of amixture comprising (a) 5 to 65 by weight of a water-soluble ammoniumsalt of a copolymer, having a molecular weight of at least about 50,000,of about 15 to 40% by weight of an acid of the formula in which n is aninteger having a volume of l to 2, and 60 to of at least one ester of anacid of the aforesaid formula with an alcohol having 1 to 2 carbon atomsand (b) 35 to 95% by weight of an alkali-soluble protein, drying, andcalendering the coated sheet.

12. A method of producing mineral-coated paper which comprises coatingat least one side of a paper sheet with an aqueous dispersion of 40 to70% total solids concentration, having a pH of 8.5 to 9.5, andcontaining a pigment having a predominant proportion of clay and 8 to25%, on the total weight of pigment, of a binder comprising, for eachparts by weight of the pigment, from 8 to 25 parts by weight of amixture comprising (a) 5 to 65% by weight of a water-soluble ammoniumsalt of a copolymer, having a molecular weight of at least about 50,000,of about 15 to 40% by weight of an acid of the formula in which n is aninteger having a value of l to 2, and 60 to 85 of at least one ester ofan acid of the aforesaid formula with an alcohol having 1 to 2 carbonatoms and (b) 35 to 95% by weight of a water-insoluble, alkaliinsolublelinear addition polymer having an apparent second order transitiontemperature of -45 to +20 C., drying, and calendering the coated sheet.

13. A method of producing mineral-coated paper which comprises coatingat least one side of a paper sheet with an aqueous dispersion of 40 to70% total solids concentration, having a pH of 8.5 to 9.5, andcontaining a pigment having a predominant proportion of clay and 8 to 25on the total weight of pigment, of a binder comprising, for each 100parts by weight of the pigment, from 8 to 25 parts by weight of amixture comprising (a) 5 to 65% by weight of a water-soluble ammoniumsalt of a copolymer, having a molecular weight of at least about 50,000,sisting of acrylic acid, methacrylic acid, and itaconic acid, of aboutto by weight of an acid of the formula and drying the coated sheet.

OH2=C (CH)MH References Cited in the file of this patent OOOH in which nis an integer having a value of 1 to 2, and 5 UNITED STATES PATENTS toof at least one ester of an acid of the aforesaid 2,754,280 Brown et y10, 1956 formula with an alcohol having 1 to 2 carbon atoms and 2759847Frost l- Allg- 1956 (b) 35 to by Weight of a water-insoluble, alkali-in-27903736 McLafghlm et a1 P 1957 soluble linear addition copolymer of 70to 93% by weight 10 236x773 MKmght et a1 23, 1958 of vinyl acetate, 5 to25% by weight of an alkyl acrylate 2874066 McLaughlm et a1 1959 in whichthe alkyl group has 1 to 2 carbon atoms, and 2 2,923,646 Jordan 0 to 8%by weight of an acid selected from the group con-

1. A MINERAL-COATED PAPER PRODUCT COMPRISING A PAPER SHEET CARRYING ON ASURFACE THEREOF OF A DRIED COATING OF A COMPOSITION COMPRISING (1) AFINELY-DIVIDED PIGMENTARY MATERIAL HAVING A PREDOMINANT PROPORTION OFCLAY AND (2) A BINDER COMPRISING, FOR EACH 100 PARTS BY WEIGHT OF THEPIGMENT, FROM 8 TO 25 PARTS BY WEIGHT OF A MIX TURE COMPRISING (A) 5 TO65% BY WEIGHT OF A WATER-SOLUBLE AMMONIUM SALT OF A COPOLYMER, HAVING AMOLECULAR WEIGHT OF AT LEAST ABOUT 50,000, OF ABOUT 15 TO 40% BY WEIGHTOF AN ACID OF THE FORMULA
 4. A MINERAL-COATED PAPER PRODUCT COMPRISING APAPER SHEET CARRYING ON A SURFACE THEREOF A DRIED COATING OF ACOMPOSITION COMORISING (1) A FINELY-DIVIDED PIGMENTARY MATERIAL HAVING APREDOMINANT PROPORTION OF CLAY AND (2) A BINDER COMPRISING, FOR EACH 100PARTS BY WEIGHT OF THE PIGMENT, FROM 8 TO 25 PARTS BY WEIGHT OF AMIXTURE COMPRISING (A) 5 TO 65% BY WEIGHT OF A WATER-SOLUBLE AMMONIUMSALT OF A COPOLYMER, HAVING A MOLECULAR WEIGHT OF AT LEAST ABOUT 50,000OF ABOUT 15 TO 40% BY WEIGHT OF AN ACID OF THE FORMULA